Mixed-resistance structured packing

ABSTRACT

A layer of mixed-resistance structured packing includes: a first structured packing having a first packing resistance; and a second structured packing generally horizontally adjacent the first structured packing, the second structured packing having a second packing resistance different than the first packing resistance. The layer of mixed-resistance structured packing is used in exchange columns for exchanging heat and/or mass between a first phase and a second phase in processes such as cryogenic air separation. Use of the layer of mixed-resistance structured packing reduces HETP (height equivalent to a theoretical plate) in the exchange columns and processes. A method also is provided for assembling the layer of mixed-resistance structured packing in an exchange column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of Ser. No. 09/213,612, filed Dec.18, 1998, now U.S. Pat. No. 6,425,574.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates to mixed-resistance structured packing andmethods for assembling such packing in an exchange column. Themixed-resistance structured packing has particular application inexchange columns, especially in cryogenic air separation processes,although it also may be used in other heat and/or mass transferprocesses that can utilize structured packing.

The term, “column”, as used herein, means a distillation orfractionation column or zone, ie., a column or zone wherein liquid andvapor phases are countercurrently contacted to effect separation of afluid mixture, such as by contacting of the vapor and liquid phases onpacking elements or on a series of vertically-spaced trays or platesmounted within the column.

The term “column section” (or “section”) means a zone in a columnfilling the column diameter. The top or bottom of a particular sectionor zone ends at the liquid and vapor distributors, respectively.

The term “packing” means solid or hollow bodies of predetermined size,shape, and configuration used as column internals to provide surfacearea for the liquid to allow mass transfer at the liquid-vapor interfaceduring countercurrent flow of two phases. Two broad classes of packingsare “random” and “structured”.

“Random packing” means packing wherein individual members do not haveany particular orientation relative to each other or to the column axis.Random packings are small, hollow structures with large surface area perunit volume that are loaded at random into a column.

“Structured packing” means packing wherein individual members havespecific orientation relative to each other and to the column axis.Structured packings usually are made of expanded metal or woven wirescreen stacked in layers or as spiral windings.

In processes such as distillation or direct contact cooling, it isadvantageous to use structured packing to promote heat and mass transferbetween counter-flowing liquid and vapor streams. Structured packing,when compared with random packing or trays, offers the benefits ofhigher efficiency for heat and mass transfer with lower pressure drop.It also has more predictable performance than random packing.

Cryogenic separation of air is carried out by passing liquid and vaporin countercurrent contact through a distillation column. A vapor phaseof the mixture ascends with an ever increasing concentration of the morevolatile components (e.g., nitrogen) while a liquid phase of the mixturedescends with an ever increasing concentration of the less volatilecomponents (e.g., oxygen). Various packings or trays may be used tobring the liquid and gaseous phases of the mixture into contact toaccomplish mass transfer between the phases.

There are many processes for the separation of air by cryogenicdistillation into its components (i.e., nitrogen, oxygen, argon, etc.).A typical cryogenic air separation unit 10 is shown schematically inFIG. 1. High pressure feed air 1 is fed into the base of a high pressurecolumn 2. Within the high pressure column, the air is separated intonitrogen-enriched vapor and oxygen-enriched liquid. The oxygen-enrichedliquid 3 is fed from the high pressure column 2 into a low pressurecolumn 4. Nitrogen-enriched vapor 5 is passed into a condenser 6 whereit is condensed against boiling oxygen which provides reboil to the lowpressure column. The nitrogen-enriched liquid 7 is partly tapped 8 andis partly fed 9 into the low pressure column as liquid reflux. In thelow pressure column, the feeds (3,9) are separated by cryogenicdistillation into oxygen-rich and nitrogen-rich components. Structuredpacking 11 may be used to bring into contact the liquid and gaseousphases of the oxygen and nitrogen to be separated. The nitrogen-richcomponent is removed as a vapor 12, and the oxygen-rich component isremoved as a vapor 13. Alternatively, the oxygen-rich component can beremoved from a location in the sump surrounding reboiler/condenser 6 asa liquid. A waste stream 14 also is removed from the low pressurecolumn. The low pressure column can be divided into multiple sections.Three such sections with packing 11 are shown in FIG. 1 by way ofexample.

The most commonly used structured packing consists of corrugated sheetsof metal or plastic foils (or corrugated mesh cloths) stackedvertically. These foils may have various forms of apertures and/orsurface roughening features aimed at improving the heat and masstransfer efficiency. An example of this type of packing is disclosed inU.S. Pat. No. 4,296,050 (Meier). It also is well-known in the prior artthat mesh type packing helps spread liquid efficiently and gives goodmass transfer performance, but mesh type packing is much more expensivethan most foil type packing.

The separation performance of structured packing is often given in termsof height equivalent to a theoretical plate (HETP). The term “HETP”means the height of packing over which a composition change is achievedwhich is equivalent to the composition change achieved by a theoreticalplate. The term “theoretical plate” means a contact process betweenvapor and liquid such that the existing vapor and liquid streams are inequilibrium. The smaller the HETP of a particular packing for a specificseparation, the more efficient the packing because the height of packingbeing utilized decreases with the HETP.

The efficiency of distillation columns with structured packing shows adependency on their diameter when all the other geometric and processfactors are held constant. While performing equivalent separations atdifferent scales, as the diameter increases from a small fraction of ameter to several meters, the HETP increases first and then tends tolevel out. This may be explained by a combination of two factors—theflow characteristics and the mixing characteristics of structuredpacking columns.

In terms of flow characteristics, even when the initial liquid and vapordistribution into a packed section of a column is highly uniform, thedistribution changes as the liquid and vapor flow in countercurrentcontact through the packed section, resulting in variations in theliquid to vapor (L/V) ratio across the cross section of the column.Also, it is known that a significant flow of liquid occurs at the columnwall, thereby reducing the liquid loading in the packing in an annularregion of the packing adjacent the wall. The vapor flow, although notcompletely uniform, is more uniform within the packing than is theliquid flow.

Thus, there usually is a systematic variation in the L/V ratio acrossthe cross section of a typical cylindrical packed column as shownschematically in FIG. 2. Referring to FIG. 2, in a typical cylindricalpacked column 22, there is an annular space 19 between the column innerwall 40 and the packing, which is disposed between the parallel brokenlines 16 (representing the perimeter of a cylindrical layer of packing).The column axis is represented by broken line 15. Broken line 17represents the “nominal” L/V ratio for theoretical or ideal conditionswhere there would be no variation in the L/V ratio across the crosssection of the column. Solid line 18 is a schematic representation ofthe non-uniform L/V ratio (relative to nominal) across the cross sectionof a typical cylindrical packed column. The L/V ratio is much highernear the column inner wall because of excessive liquid flowing down thecolumn inner wall (as indicated by the steep slope of line 18 aboveannular space 19 in FIG. 2).

The general pattern of the actual L/V ratio illustrated by line 18 inFIG. 2 may vary considerably depending on the details of the packing,the mixture being separated, and the process conditions.

Further, it is well known that maldistribution can result in degradationof the separation efficiency of the column unless it is mitigated byrepeated mixing of the liquid and vapor phases within the column. Thisis especially true for tight separations such as those used in cryogenicair separation.

In terms of mixing characteristics, a small diameter column with a largelength to diameter (l/d) ratio (e.g., about 5 to 20) can mix the vaporflow and, to a lesser extent, the countercurrent liquid flow repeatedlyacross the column cross section, which can average out the consequencesof local variations in L/V ratios much better than a large diametercolumn with a much lower l/d ratio (e.g., about 0.5 to 5.0). For thisreason, the degradation in separation efficiency compared to the idealis more severe in large diameter columns, which results in an increasein HETP.

The increase in HETP in large exchange columns is a major economicpenalty, since it increases the overall height of the system of whichthe column is a part. It is desired to mitigate the increase in HETP inlarge diameter columns, so that such columns may approach theperformance of small diameter columns in terms of separation efficiency.

The prior art has not recognized or addressed this specific problem. Theprior art has recognized the deleterious effect of excessive wall liquidflow, and there have been attempts to mitigate that effect, such as bythe use of conventional wall wipers. However, although wall wipers canreduce wall liquid flow locally, wall wipers are not very effective inreturning liquid back to the packing. Thus, even in columns equippedwith wall wipers, there still are unfavorable variations in L/V ratios.The deleterious effect of vapor bypass at the column wall can bemitigated by the use of restricting means in the annular space near thecolumn wall, such as the solid metal wipers and other devices disclosedin the following copending U.S. patent application assigned to theassignee of the instant application: Ser. No. 09/166373 to Klotz, et al.entitled “Devices to Minimize Vapor Bypass in Packed Column and Methodof Assembly”, now abandoned.

U.S. Pat. No. 5,262,095 (Bosquain et al.) describes the use of packingedge modification by deformation, slits, porous plugs, fillers orspecial wipers in order to promote a flow reversal of liquid back intothe packing and away from the wall of the column. U.S. Pat. No.5,441,793 (Suess) describes the use of liquid re-director elements atthe packing edges near the wall. Such elements may be made out of “L”shaped mini corrugations. U.S. Pat. No. 5,224,351 (Jeannot et al.)describes similar edge modifications by folding some of the corrugationedges near the column wall. U.S. Pat. No. 5,700,403 (Billingham et al.)describes the formation of special corrugated packing layers whereinalternate corrugated elements within a structured packing layer near thewall are cut short so that the tendency to lead liquid towards the wallis reduced. U.S. Pat. No. 5,282,365 (Victor et al.) describes the use ofheat addition at the column wall in order to vaporize and reduce wallflow.

While the packings and methods taught in the first four patents mayreduce wall liquid flow, the associated costs are expensive, since themanufacturing techniques are unconventional and installation of thepackings would likely be labor intensive. The proposed solution of thefifth patent also would be expensive, because it would requireadditional process circuitry to bring another fluid outside the columnin order to evaporate the wall liquid inside the distillation equipment.

U.S. Pat. No. 5,100,448 (Lockett et al.) discloses the use of structuredpacking of different packing density in at least two sections of acolumn which are directly above and below each other to balancehydraulic loading. Likewise, in U.S. Pat. No. 5,419,136 (McKeigue) thecorrugation angle of the structured packing is varied in two sectionswhich are directly above and below each other for the purpose ofbalancing hydraulic loading. Although these arrangements of packingreportedly provide improved operating flexibility in cryogenic airseparation, they do not address the problems of maldistributiondiscussed herein, nor do they provide a solution or a suggestion of asolution for any of those problems.

It is desired to have a structured packing which minimizes the effectsof maldistribution using a variation of conventional structured packingwhich does not require any special edge modification of the packing orany additional equipment or circuitry outside the exchange column.

It is further desired to have a structured packing that shows highperformance characteristics for cryogenic applications, such as thoseused in air separation, and for other heat and/or mass transferapplications. Specifically, it is desired to mitigate the increase inHETP in large diameter columns used in such applications, so that suchcolumns approach the performance of small diameter columns in terms ofseparation efficiency.

It is still further desired to have an exchange column wherein theoverall liquid to vapor (L/V) ratio within the column deviates as littleas possible from the nominal (excluding wall effects), thereby resultingin an improved mass transfer performance.

It is still further desired to have an exchange column having astructured packing wherein the L/V ratio is maintained nearly constantin the column even if the absolute liquid and vapor flows are notmaintained constant.

It is still further desired to balance the L/V ratio across the crosssection of an exchange column and to make large diameter columnsapproach the performance of small diameter columns in mass and/or heattransfer efficiency.

BRIEF SUMMARY OF THE INVENTION

The present invention is a layer of mixed-resistance structured packing,which may be used in one or more sections of an exchange column forexchanging heat and/or mass between a first phase and a second phase ina process, such as cryogenic air separation. The invention also providesa method for assembling such a layer of mixed-resistance structuredpacking in an exchange column. Other aspects of the invention are amethod and a system for reducing HETP (height equivalent to atheoretical plate) in exchange columns.

The mixed-resistance structured packing may be used in one or morelayers of packing in one or more sections of an exchange column. In sucha layer of mixed-resistance structured packing, lower resistance packingis used in the central core and a higher resistance packing is used inan outer annulus surrounding the central core. This forces more vaporflow toward the center of the exchange column and less toward the columnwall, thereby counteracting a tendency of liquid to maldistribute in theexchange column. By using the method of the present invention to balancethe L/V ratio (liquid to vapor ratio), large diameter columns approachthe performance of small diameter columns in terms of substantiallylower HETP.

In one embodiment, the layer of mixed-resistance structured packingcomprises: a first structured packing having a first packing resistance;and a second structured packing generally horizontally adjacent thefirst structured packing, the second structured packing having a secondpacking resistance different than the first packing resistance.

In one variation, the second structured packing has an inner perimetersubstantially equal to the outer perimeter of the first structuredpacking and an outer perimeter greater than the inner perimeter. Theinner perimeter of the second structured packing substantially abuts theouter perimeter of the first structured packing. In another variation,the outer perimeter of the first structured packing and the innerperimeter of the second structured packing are substantially circular.

In another variation, the first and second structured packings compriseat least one corrugated plate. In yet another variation, the first andsecond structured packings comprise a plurality of corrugated platesmade of foil-like material disposed in parallel relation, each saidplate having at least one corrugation disposed at an angle and in acrisscrossing relation to at least one corrugation of an adjacent plate.A difference in resistance between the first and second structuredpackings may be due to a difference in the angles of the corrugations.For example, the angle of the at least one corrugation in the firststructured packing may be different than the angle of the at least onecorrugation in the second structured packing.

In yet another variation, a difference in resistance between the firstand second structured packings is due to a difference in surface areadensity of the first and second structured packings. For example, thesurface area density of the second structured packing may exceed thesurface area density of the first structured packing.

Another embodiment of the invention is a layer of mixed-resistancestructured packing comprising: a substantially circular central corehaving an outer perimeter, the central core comprising a firststructured packing having a first packing resistance; and an outerannulus generally horizontally adjacent the outer perimeter of the outercore, the outer annulus comprising a second structured packing having asecond packing resistance different than the first packing resistance.

Another aspect of the present invention is an exchange column forexchanging heat and/or mass between a first phase and a second phase,the exchange column having at least one layer of mixed-resistancestructured packing as in any one of the embodiments or variationsdescribed above.

Yet another aspect of the present invention is a process for cryogenicair separation comprising contacting vapor and liquid counter-currentlyin at least one distillation column containing at least one masstransfer zone wherein liquid-vapor contact is established by at leastone layer of mixed-resistance structured packing as in any of theembodiments and variations described above.

The present invention also includes a method for assembling a layer ofmixed-resistance structured packing in an exchange column comprisingmultiple steps. The first step is to provide an exchange column. Thesecond step is to provide a layer of mixed-resistance structuredpacking, the layer of mixed-resistance structured packing comprising: afirst structured packing having a first packing resistance; and a secondstructured packing generally horizontally adjacent the first structuredpacking, the second structured packing having a second packingresistance different from the first packing resistance. The final stepis to install the layer of mixed-resistance structured packing in theexchange column.

Another aspect of the present invention is a method for reducing HETP(height equivalent to a theoretical plate) in an exchange column forexchanging heat and/or mass between a liquid and a vapor, the exchangecolumn having at least one layer of structured packing, the layer ofstructured packing having a central core and an outer annulus generallyhorizontally adjacent the central core. The method comprises thefollowing steps: inducing at least a portion of the vapor in theexchange column away from the outer annulus; and inducing the at least aportion of the vapor toward the central core. In one variation of themethod for reducing HETP, the portion of the vapor is an amount wherebythe liquid-vapor ratio across a cross section of the exchange column ismaintained at nearly a constant value.

Yet another aspect of the invention is a system for reducing HETP in anexchange column for exchanging heat and/or mass between a liquid and avapor, the exchange column having at least one layer of structuredpacking, the layer of structured packing having a central core and anouter annulus generally horizontally adjacent the central core. Thesystem comprises: means for inducing at least a portion of a vapor inthe exchange column away from the outer annulus; and means for inducingthe at least portion of the vapor toward the central core. In onevariation of the system, the portion of the vapor is an amount wherebythe liquid-vapor ratio across a cross section of the exchange column ismaintained at nearly a constant value.

Another aspect of the present invention is a packed section in anexchange column comprising: a first layer of mixed-resistance structuredpacking (as in any one of the embodiments or variations describedabove); and a second layer of mixed-resistance structured packing (as inany one of the embodiments or variations described above) located belowthe first layer of mixed-resistance structured packing, wherein thesecond layer is rotated at an angle relative to the first layer. Theangle may be between about 0° and about 90°.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic illustration of an air separation unit;

FIG. 2 illustrates the L/V ratio non-uniformity in a typical cylindricaldistillation column using conventional structured packing;

FIG. 3 is a schematic diagram of a plan view of a layer ofmixed-resistance structured packing in an exchange column;

FIG. 4A is a perspective view of a conventional structured packingelement;

FIG. 4B is a schematic diagram illustrating the crisscrossingarrangement of adjacent elements in conventional structured packing;

FIG. 4C is a schematic diagram illustrating the use of wall wipers in apacked column;

FIG. 5A is a schematic diagram of a plan view of an arrangement ofbricks of structured packing at one elevation at a sectional view takenalong line 5A—5A in FIG. 5B;

FIG. 5B is a schematic diagram of an elevation view of an arrangement ofa plurality of layers of structured packings between liquid and vapordistributors in a section of a distillation column;

FIG. 6 is a schematic diagram illustrating the flows of liquid and vaporin a low pressure column of a conventional two-column air separationunit;

FIG. 7 is another schematic diagram illustrating the flows of liquid andvapor in a low pressure column in a two-column air separation unit; and

FIG. 8 is a chart illustrating the effect of liquid maldistribution andmixing on two parallel columns with or without balancing vapordistribution.

DETAILED DESCRIPTION OF THE INVENTION

For ease of discussion, the present invention is described usingconventional structured packing elements, bricks, and layers illustratedin FIGS. 4A through 5B and discussed below. However, the invention alsomay be used with other types of structured packing, including but notlimited to the types of packings disclosed in the following copendingU.S. patents assigned to the assignee of the instant application: U.S.Pat. No. 6,119,481 to Sunder entitled “Horizontal Structured Packing”;U.S. Pat. No. 5,901,575 to Sunder entitled “Stackable Structured Packingwith Controlled Symmetry”; and U.S. Pat. No. 5,876,638 to Sunderentitled “Structured Packing Element with Bi-Directional Surface Textureand a Mass and Heat Transfer Process Using Such Packing Element”.

Referring to FIG. 3, the present invention is discussed with referenceto a layer of structured packing 20 within a section of a packed column22 wherein the layer includes a higher resistance packing 24 (identifiedby “A”) and a lower resistance packing 26 (identified by “B”) in theconfiguration illustrated in FIG. 3. (As discussed below and illustratedin FIGS. 4A through 5B, a “layer” is typically made from a plurality of“bricks” of packing elements or sheets that fit together to fill thecross section of a column.) As shown in FIG. 3, the higher resistancepacking (“A”) is in the outer annulus 28 and the lower resistancepacking (“B”) is in the central core 30.

An “annulus” is defined as the portion of a plane bounded by twoconcentric circles in the plane. As used herein for the embodiment shownin FIG. 3, the term “outer annulus” 28 is the portion of the planebounded by the substantially circular inner wall 40 of the column 22 andthe concentric circle 42 defining the perimeter of the “central core”30.

Although the boundary between the central core 30 and the outer annulus28 is illustrated as a circle 42 in FIG. 3, for manufacturing reasonsthe boundary in actual practice is a jagged boundary having a series ofstraight lines roughly approximating circle 42. The segmentation of thelayer illustrated in FIG. 3 is only one example. Many other variationsare possible, depending on the column diameter and the packingdimensions.

The invention is not limited to the configuration shown in FIG. 3, noris it limited to the use of only two packings of two differentresistance levels. Persons skilled in the art will recognize that otherconfigurations may be used and that more than two different packingshaving different resistances may be used.

As illustrated in FIG. 3, the relative sizes of the outer annulus 28 andthe central core 30 are determined by the diameter of circle 42 (i.e.,the perimeter of the central core). For a given size column 22, if thecentral core 30 is relatively larger (i.e., circle 42 has a largerdiameter) than that shown in FIG. 3, then the outer annulus 28 will berelatively smaller than that shown in FIG. 3. Conversely, if the centralcore is relatively smaller (i.e., circle 42 has a smaller diameter) thanthat shown in FIG. 3, then the outer annulus will be relatively largerthan that shown in FIG. 3. Persons skilled in the art will recognizethat numerous variations are possible, as the diameter of circle 42 canrange from a lower limit near zero to an upper limit near the diameterof inner wall 40.

Although the preferred embodiment utilizes a circular central core 30 ina packed column 22 having a circular inner wall 40, other combinationsare possible. For example, the shapes of the inner wall 40 of column 22and/or the perimeter 42 of the central core 30 may be other thancircular. In those cases, the “outer annulus” 28 would not have theactual geometric shape of an “annulus” as defined above. Instead, theouter annulus would be in the form of a geometric shape having an outerperimeter defined by inner wall 40 and an inner perimeter defined by theouter perimeter 42 of the central core 30.

Referring to the embodiment illustrated in FIG. 3, the present inventionworks by counteracting the depletion of liquid flow in the outer annulus28 of the packing by inducing a similar reduction in the vapor flowwithin the outer annulus. This can be accomplished by mixing resistancesin such a way that the outer annulus of some or all layers within asection has a higher resistance packing 24 and the central core 30 has alower resistance packing 26, wherein the term resistance refersprimarily to the resistance relative to the vapor flow.

Mixed resistances may be obtained by varying one or more of thefollowing variables between the central core 30 and the outer annulus28—corrugation angle, surface area density, surface texture,perforations, packing types, or other variables which change theresistance characteristics of the packing.

By inducing less vapor flow in the outer annulus 28 and more vapor flowtoward the central core 30, the overall liquid to vapor (L/V) ratiowithin the column 22 shows less deviation from the nominal and therebythe mass transfer performance improves in terms of the required HETP.For commercial columns, reductions in heights can reduce the overallpressure drop in spite of increased resistance in the outer annulus.With the resulting balancing of the liquid to vapor flow (L/V) ratios,at least some of the degradation in the HETP of large industrial columnscan be reclaimed. This can result in much lower section heights, whichtranslates into a reduction in the overall cost of a system.

The conventional technology for using structured packing has beendescribed in many of the patents pertaining to structured packing thatfollowed U.S. Pat. No. 4,296,050 (Meier), which describes corrugatedstructured packing and its applications. A basic conventional structuredpacking element 32 is shown in FIG. 4A. Each packing element is made ofthin metal foil or other suitable material which is corrugated. Adistillation column 22 packed with conventional structured packing isillustrated in FIGS. 5A and 5B.

A typical structured packing employs vertically-oriented corrugatedpacking sheets or elements such as that in FIG. 4A wherein thecorrugations are arranged at an angle to the vertical. Each packingsheet is positioned such that its corrugation direction is reversed fromthe corrugation direction of its adjacent packing sheet, as illustratedin FIG. 4B. (The solid diagonal lines represent the corrugations of onepacking sheet, and the broken diagonal lines represent the corrugationsof an adjacent packing sheet.) When placed in the vertical for use in adistillation column the corrugations form an angle (∝) with thehorizontal. In addition to being corrugated, the elements or sheets mayhave surface texture, holes or other orifices, dimples, grooves, orother features which can enhance the performance of the basic element.

Using such basic packing elements, a “brick” 34 of structured packing ismade by assembling the elements (typically about 40 to 50 elements perbrick) such that the corrugations of adjacent elements are arranged inthe crisscrossing fashion shown in FIG. 4B. (The means used to securethe elements in place are not shown.) When the bricks are placed withina cylindrical column, the edges of the bricks near the wall are roughand jagged, creating gaps. To reduce liquid bypass, wipers 36 typicallyare used as shown in FIG. 4C.

Structured packing bricks 34 typically are assembled into layers (48,48′) in a section of a distillation column 38 as shown in FIGS. 5A and5B. FIG. 5A is a plan view which shows the arrangement of about twelvebricks at one elevation as a sectional view at 5A—5A in FIG. 5B. FIG. 5Bshows an elevation view of the entire arrangement of a structuredpacking column having a plurality of layers (48, 48′) in a sectionbetween a liquid distributor 44 and a vapor distributor 46, whereinsuccessive layers (48, 48′) of packing (typically about 8 inches highper layer) are rotated relative to each other at right angles (i.e.,90°). This is the most common arrangement, but other rotation patternscan be used (e.g., where successive layers are rotated at an anglebetween about 0° and about 90°).

The present invention modifies the arrangement of conventional packing,as discussed below. In conventional packing, the bricks 34 at oneelevation, such as shown in FIG. 5A, are all formed from identicalelements 32, as shown in FIG. 4A. The present invention uses at leasttwo different types of the basic elements, depending on the location ofthe bricks, as shown in FIG. 3. Those located in the outer annulus 28are formed of elements that provide a higher resistance to vapor flow,and those in the central core 30 are formed of elements that provide alower resistance to vapor flow.

The differences in resistance may be due to differences in the surfacearea density, which usually is expressed in terms of m²/m³ of the volumeoccupied by the packing. (The term “surface area density” means thesurface area of the structured packing per unit volume of the structuredpacking.) Thus, the surface area density of the packing used in theouter annulus 28 would be higher relative to the surface area density ofthe packing in the central core 30.

Alternatively, while retaining the same surface area density in both theouter annulus 28 and the central core 30, a packing having a lowercorrugation angle could be used in the outer annulus 28 relative to thecorrugation angle of a packing in the central core 30. In addition,other surface features also may be mixed to differentiate the packingsbetween the outer annulus and the central core. These features includetextures, holes or orifices, dimples, grooves, shapes of thecorrugations, waves, or other means which may be mixed singularly orsimultaneously in combination with other features to achieve the purposeof varying the resistance to vapor flow between the outer annulus andthe central core.

Other variations and extensions of these concepts will be obvious topersons skilled in the art. For example, variations could include morethan two resistances in each layer in several sequential annularsections, or application of mixed resistances in only some, but not all,layers of a packed section. This general technique also may be appliedto any heat and mass exchange column which has counter flowing liquidand vapor (or gas phases) and which exhibits systematic maldistributionsuch as described above. The present invention also is not limited todistillation or to cryogenic distillation applications.

The present invention is explained further by the analysis below.Although the present invention has more general applicability, for easeof discussion of the analysis, the analysis refers to the separation ofargon and oxygen in a conventional two-column air separation plant.

Sample calculations of the consequences of imbalance of the L/V ratioand the corrective effect of the current invention are provided below.The separation example represents the bottom of a low pressure column 4in a conventional two-column air separation plant 10, such as that shownin FIG. 1.

The assumed conditions for the calculations are shown in FIG. 6. Inaddition, the following parameters were assumed: argon/oxygen mixture;25 psia; 25 theoretical stages; nominal L/V=1.4. Calculations were firstperformed to compute the ideal separation under uniform flow conditions.Those results were then compared with a separation when the column issplit into two parallel columns of equal area having different L/Vratios within the columns. As some mixing occurs in real columns, thelevel of mixing was studied as a variable between 0 and 3 intermediatemixes.

An example of a specific mixing pattern is shown schematically in FIG.7. (Other mixing patterns may be obtained in a similar manner.) FIG. 7provides an example of relative flows, assuming only liquidmaldistribution with one intermediate mix. The relative flow splits forliquid are indicated in the upper portion of the diagram, and therelative flow splits for vapor are indicated in the lower portion of thediagram.

The consequences of the present invention were calculated by showing theeffect of rebalancing the L/V ratios in the two parallel columns,although with different absolute flows. The parameters used in thesecalculations were: Ar/O₂ binary, 25 psia, L/V 1.4, 7% Ar in liquid atthe top, 0.5% Ar at the bottom and 25 theoretical stages. The results ofthe calculations are shown in FIG. 8 and tabulated below.

Calculated relative packed column lengths to accomplish the sameseparation. Maldistri- L L L L&V L&V L&V bution: maldist maldist maldistmaldist maldist maldist +/−% 0 mixes 1 mix 2 mixes 0 mixes 1 mix 2 mixes0.0 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 2.6 1.0539 1.0000 5.01.2880 1.0452 1.0003 1.0003 10.0  1.2400 1.0928 1.0015 1.0015 15.0 1.2650 1.0030

The findings from these calculations are as follows. First, the HETP ofan ideal column stated in relative terms is 1. This is shown in FIG. 8by all the cases which have 0 maldistribution within the two parallelcolumns. The number of intermediate mixes has no effect on thiscalculation. But when liquid maldistribution is imposed at ±2.6%, 5%,10% and 15% in the two columns relative to the mean, the overall HETPincreases. The graphed results in FIG. 8 show that the relative HETPincreases with increasing level of liquid maldistribution. Whileintermediate mixing mitigates this effect, it does not eliminate it. Incontrast, providing a counterbalancing vapor maldistribution to restorethe L/V ratios between the two parallel columns practically eliminatesthe problem. For instance, if the liquid maldistribution is ±5%, thenHETP increases to a relative value of 1.288. One (1) intermediate mixreduces this to only 1.045. However, if counterbalancing vapor flow isinduced in the same proportion in order to restore the L/V ratio to 1.4in both columns, then the HETP goes back down to 1.0003, even with nomixing. Similarly, with ±15% liquid maldistribution, even with 2intermediate mixes, relative HETP increases to 1.265. Restoring L/Vratio by counter balancing vapor flow reduces the HETP to 1.003.

The calculations show that it is very important to maintain the L/Vratio nearly constant in a distillation column even if the absoluteliquid and vapor flows cannot be maintained constant. The calculationsalso show that intermediate mixing mitigates the effects ofmaldistribution. But as mixing is increasingly limited when the columndiameter increases, a secondary means, such as taught by the presentinvention, can improve the performance of the column.

It should be noted that the above calculations were performed withseveral specific assumptions as an example only. If changes are made inthe specific mixture, or the process conditions, or the maldistributionand mixing patterns, the results will show the same qualitative trendseven though there may be changes in quantitative terms. Thus, thepresent invention has very general applicability to contact towers thathave liquid and gas or vapor flowing in countercurrent directions. Itcan apply to cryogenic and non-cryogenic distillation, as well as anyheat exchange and/or mass exchange operation which uses structuredpacking as the contacting means.

The present invention uses a variation with existing conventionalstructured packing which does not require any special edge modificationof the packing within the column or any additional equipment orcircuitry outside the column. Also, rather than attempting to eliminatethe liquid flow variation, it counters that variation by inducing asimilar flow variation in the vapor flow such that the L/V ratiovariation is minimized.

The concept of using mixed resistances within a single layer so thatthere are differences in resistance between the central core and theouter annulus of a column has not been suggested in the prior art. It iscustomary to use the same packing within the entire packed section. Theprior art does use different packings in completely separate packedsections within a packed column, which can be done by varying surfacearea, corrugation angle, or other means. (For example, see U.S. Pat. No.5,100,448 (Lockett et al.)). But that is completely different from thepresent invention in structural arrangement, as well as in the intendedpurpose. The purpose of those prior art arrangements is to get evenapproach to flooding between different sections of a distillationcolumn, while the purpose of the current invention is to balance L/Vratios across the cross section of a column and to make large columnsapproach small columns in mass and/or heat transfer efficiency.

Various embodiments of the present invention have been described withparticular reference to the examples illustrated. However, it should beappreciated that variations and modifications may be made to thoseembodiments and examples without departing from the spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A mixed resistance structured packing comprisinga plurality of layers of packing for an exchange column, each of saidlayers comprising: a central core having an outer perimeter, the centralcore comprising a first structured packing having a first packingresistance; and an outer annulus horizontally adjacent the outerperimeter of the central core, the outer annulus comprising a secondstructured packing having a second packing resistance higher than thefirst packing resistance; whereby vapor flow is increased toward thecenter of the exchange column and decreased toward a column wall of theexchange column in all of said plurality of layers.
 2. Amixed-resistance structured packing as in claim 1, wherein: the firststructured packing has an outer perimeter; the second structured packinghas an inner perimeter substantially equal to the outer perimeter of thefirst structured packing and an outer perimeter greater than the innerperimeter; and the inner perimeter of the second structured packingsubstantially abuts the outer perimeter of the first structured packing.3. A mixed-resistance structured packing as in claim 2, wherein theouter perimeter of the first structured packing and the inner perimeterof the second structured packing are substantially circular.
 4. A mixedresistance structured packing comprising a plurality of layers ofpacking for an exchange column, each of said layers comprising: asubstantially circular central core having an outer perimeter, thecentral core comprising a first structured packing having a firstpacking resistance; and an outer annulus generally horizontally adjacentthe outer perimeter of the central core, the outer annulus comprising asecond structured packing having a second packing resistance higher thanthe first packing resistance; whereby vapor flow is increased toward thecenter of the exchange column and decreased toward a column wall of theexchange column in all of said plurality of layers.
 5. Amixed-resistance structured packing as in claim 1, wherein the first andsecond structured packings comprise at least one corrugated plate.
 6. Amixed resistance structured packing comprising a plurality of layers ofpacking for an exchange column, each of said layers comprising: acentral core having an outer perimeter, the central core comprising afirst structured packing having a first packing resistance; and an outerannulus horizontally adjacent the outer perimeter of the central core,the outer annulus comprising a second structured packing having a secondpacking resistance higher than the first packing resistance, wherein thefirst and second structured packings comprise a plurality of corrugatedplates made of foil-like material disposed in parallel relation, eachsaid plate having at least one corrugation disposed at an angle and incrisscrossing relation to at least one corrugation of an adjacent plate.7. A mixed-resistance structured packing as in claim 6, wherein theangle of the at least one corrugation in the first structured packing isdifferent than the angle of the at least one corrugation in the secondstructured packing.
 8. A mixed-resistance structured packing as in claim1, wherein a surface area density of the second structured packingexceeds a surface area density of the first structured packing.
 9. Anexchange column for exchanging heat and/or mass between a first phaseand a second phase, the exchange column having the mixed-resistancestructured packing as in claim
 1. 10. A packed section in an exchangecolumn, comprising: a first layer of the plurality of layers of themixed-resistance structured packing as in claim 1; and a second layer ofthe plurality of layers of the mixed-resistance structured packing as inclaim 1 located below the first layer of mixed-resistance structuredpacking, wherein the second layer is rotated at an angle relative to thefirst layer.
 11. A packed section as in claim 10, wherein the angle isbetween about 0° and about 90°.